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Some study designs are better than others for answering clinical questions, we can think of evidence being arranged in a hierarchy. The hierarchy is pyramid-shaped because as the study design becomes more rigorous, fewer of these types of studies are created. The most robust evidence, meta-analysis and systematic review, are at the very top of the evidence pyramid, because they are the most intensively reviewed.

• Meta-Analysis: A systematic review that uses quantitative methods to summarize the results.

• Systematic Review: Authors have systematically searched for, appraised, and summarized all of the medical literature for a specific topic.

• Critically Appraised Topic: Authors evaluate and synthesize multiple research studies.

• Critically Appraised Articles: Authors evaluate and synopsize individual research studies.

• Randomized Controlled Trials: Include a randomized group of patients in an experimental group and a control group. These groups are followed up for the variables/outcomes of interest.

• Cohort Study: Identifies two groups (cohorts) of patients, one which did receive the exposure of interest, and one which did not, and following these cohorts forward for the outcome of interest.

• Case-Control Study: Identifies patients who have the outcome of interest (cases) and control patients without the same outcome, and looks for exposure of interest.

• Background Information/Expert Opinion: Handbooks, encyclopedias, and textbooks often provide a good foundation or introduction and often include generalized information about a condition. While background information presents a convenient summary, often it takes about three years for this type of literature to be published. Expert opinions are often found in scholarly sources as Review Articles (NOTE- these are different than Systematic Reviews).

"Evidence-based nursing (EBN) means using the best available evidence from research, along with patient preferences and clinical experience, when making nursing decisions." 

 Cullum N. (2000) Users' guides to the nursing literature: an introduction. Evidence Based Nursing.  Jul;3(3):71-2. doi:10.1136/ebn.3.3.71

"An ongoing process by which evidence, nursing theory and the practitioners’ clinical expertise are critically evaluated and considered, in conjunction with patient involvement, to provide delivery of optimum nursing care for the individual."

Scott, K. & McSherry, R. "Evidence Based Nursing: clarifying the concepts for nurses in practice." Journal of Clinical Nursing 2009; 18(8): 1085-95.

Steps in Evidence Based Practice are:

1. Asking answerable questions
2. Finding the best evidence
3. Critically appraising the evidence
4. Applying a decision integrating the evidence with clinical expertise and patient preferences and values
5. Evaluating outcomes or changes
6. Disseminating results, within team, department, community, etc.

Authors of a systematic review;

• ask a specific clinical question,
• perform a comprehensive literature search,
• eliminate the poorly done studies
• and attempt to make practice recommendations based on the well-done studies.

A meta-analysis is a systematic review that combines all the results of all the studies into a single statistical analysis of results.

Evidence summaries & evidence guidelines

Evidence is not always available via the resources and search strategies above. Instead, searching the primary literature may be required. It is possible to use specific search strategies in PubMed MEDLINE and CINAHL to achieve the highest possible level of evidence.

Different types of research studies are better suited to answering questions in various domains of inquiry. ALL research questions are best answered by well-done Meta-Analysis or Systematic Reviews; however, sometimes they are not available. In those cases, here are suggestions for types of single study by domain.

Domain— Single Study Types

Therapy— Randomized Controlled Trials (RCTs)

Prevention— Randomized Controlled Trials (RCTs) > cohort > case control > case series

Etiology/Harm— Cohort > case control > case series

Prognosis— Cohort (follow-up) > case control > case series

Diagnostic— Prospective, blind comparison to gold standard